In a mobile communication system, a user equipment (UE) may receive information from a base station (BS) in downlink and transmit information in uplink. The information transmitted or received by the UE includes data and a variety of control information, and a physical channel varies according to the type of information transmitted or received by the UE.
FIG. 1 is a view showing physical channels used for a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, which is an example of a mobile communication system, and a general signal transmission method using the same.
When a UE is powered on or when the UE newly enters a cell, the UE performs an initial cell search operation such as synchronization with a BS in step S101. In order to perform the initial cell search, the UE may receive a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the BS so as to perform synchronization with the BS, and acquire information such as a cell ID. Thereafter, the UE may receive a physical broadcast channel from the BS and acquire broadcast information in the cell. Meanwhile, the UE may receive a Downlink Reference signal (DL RS) in the initial cell search step and confirm a downlink channel state.
The UE, upon completes the initial cell search, may receive a Physical Downlink Control Channel (PDCCH) and a Physical Downlink Shared Channel (PDSCH) corresponding to the PDCCH, and acquire more detailed system information in step S102.
Meanwhile, if the UE does not complete access to the BS, the UE may perform a random access procedure in steps S103 to S106, in order to complete access to the BS. In order to perform a random access procedure, the UE may transmit a feature sequence via a Physical Random Access Channel (PRACH) as a preamble (S103), and may receive a response message to the random access procedure via the PDCCH and the PDSCH corresponding thereto (S104). In contention-based random access, except for handover, a contention resolution procedure including transmission of an additional PRACH (S105) and reception of the PDCCH and the PDSCH corresponding thereto (S106) may be performed.
The UE, having performed the above-described procedure, may then receive the PDCCH/PDSCH (S107) and transmit a Physical Uplink Shared Channel (PUSCH)/Physical Uplink Control Channel (PUCCH) (S108), as a general uplink/downlink signal transmission procedure.
FIG. 2 is a view explaining a signal processing procedure for transmitting an uplink signal at a UE.
In order to transmit the uplink signal, a scrambling module 201 of the UE may scramble a transmitted signal using a UE-specific scrambling signal. The scrambled signal is input to a modulation mapper 202 so as to be modulated into complex symbols using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) or 16-Quadrature amplitude modulation (QAM) according to the kind of the transmitted signal and/or the channel state. Thereafter, the modulated complex symbols are processed by a transform precoder 203, and the processed complex symbols are input to a resource element mapper 204. The resource element mapper 204 may map the complex symbols to time-frequency resource elements used for actual transmission. The signal processed as described above may be transmitted to a BS via an SC-FDMA signal generator 205 and an antenna.
FIG. 3 is a view explaining a signal processing procedure for transmitting a downlink signal at a BS.
In the 3GPP LTE system, the BS may transmit one or more codewords in downlink. Accordingly, one or more codewords may be processed by scrambling modules 301 and modulation mappers 302 to configure complex symbols, similar to the uplink transmission of FIG. 2. Thereafter, the complex symbols are mapped to a plurality of layers by a layer mapper 303, and each layer may be multiplied by a predetermined precoding matrix, which is selected according to the channel state, by a precoding module 304 and may be allocated to each transmission antenna. The processed signals which will respectively be transmitted via antennas may be mapped to time-frequency resource elements used for transmission by resource element mappers 305, and may respectively be transmitted via OFDM signal generators 306 and antennas.
In a mobile communication system, in a case where a UE transmits a signal in uplink, a Peak-to-Average Ratio may be more problematic than the case where a BS transmits a signal in downlink. Accordingly, as described above with reference to FIGS. 2 and 3, downlink signal transmission uses an OFDMA scheme, while uplink signal transmission uses an SC-FDMA scheme.
FIG. 4 is a diagram explaining an SC-FDMA scheme for uplink signal transmission and an OFDMA scheme for downlink signal transmission in a mobile communication system.
A UE for uplink signal transmission and a BS for downlink signal transmission are identical in that a serial-to-parallel converter 401, a subcarrier mapper 403, an M-point Inverse Discrete Fourier Transform (IDFT) (or IFFT) module 404 and a Cyclic Prefix (CP) attaching module 406 are included.
The UE for transmitting a signal using an SC-FDMA scheme further includes a parallel-to-serial converter 405 and an N-point DFT module 402. The N-point. DFT module 402 partially offsets an IDFT (or IFFT) process influence of the M-point IDFT (or IFFT) module 404 such that the transmitted signal has a single carrier property.
FIG. 5 is a diagram explaining a signal mapping scheme in a frequency region satisfying the single carrier property in the frequency region. FIG. 5(a) shows a localized mapping scheme and FIG. 5(b) shows a distributed mapping scheme. In the current 3GPP LTE system, a localized mapping scheme is defined.
A clustered SC-FDMA scheme which is a modified form of the SC-FDMA scheme will now be described. In the clustered SC-FDMA scheme, DFT process output samples are sequentially divided into sub-groups and are mapped to subcarrier regions which are separated from each other on a per sub-group basis in an IFFT sample input unit in a subcarrier mapping process between a DFT process and an IFFT process. In some cases, a filtering process and a cyclic extension process may be included.
At this time, a sub-group may be called a cluster and cyclic extension means that a guard interval longer than maximum delay spread of a channel is inserted between contiguous symbols in order to prevent inter-symbol interference (ISI) while each subcarrier symbol is transmitted via a multi-path channel.